Aluminum beverage containers are generally made in two pieces, one piece forming the container sidewalls and bottom (referred to herein as a "container body") and a second piece forming a container top. Container bodies are formed by methods well known in the art. Generally, the container body is fabricated by forming a cup from a circular blank aluminum sheet (i.e., body stock) and then extending and thinning the sidewalls by passing the cup through a series of dies having progressively smaller bore sizes. This process is referred to as "drawing and ironing" the container body. The upper end of the container is formed from end stock and attached to the container body. A bendable tab on the upper end of the container is used to provide an opening to dispense the contents of the container and is formed from tab stock.
Aluminum alloy sheet is most commonly produced by an ingot casting process. In the process, the aluminum alloy material is initially cast into an ingot, for example, having a thickness ranging from about 20 to about 30 inches. The ingot is then homogenized by heating to an elevated temperature, which is typically 1075.degree. F. to 1150.degree. F., for an extended period of time, such as from about 6 to about 24 hours. As used herein, "homogenization" refers to a process whereby ingots are raised to temperatures near the solidus temperature and held at that temperature for varying lengths of time. The purposes of homogenization are to (i) reduce microsegregation by promoting diffusion of solute atoms within the grains of aluminum and (ii) improve workability. The homogenized ingot is then hot rolled in a series of passes to reduce the thickness of the ingot. Homogenization does not alter the crystal structure of the ingot. The hot rolled sheet is then cold rolled to the desired final gauge.
Another method for producing aluminum alloy sheet is by continuously casting molten metal to form a cast strip. The cast strip is a relatively long, thin slab. The cast strip is then hot rolled and cold rolled to produce aluminum alloy sheet.
Some alloys are not readily cast using a continuous casting process into an aluminum sheet that is suitable for forming operations, especially for making drawn and ironed container bodies. By way of example, some 5000 series alloys have striated grain structures which lead to a number of problems. During fabrication, the aluminum alloy sheet produced from such alloys can generate unacceptably high amounts of fine particles, which cause a significant increase in the rate of wear to fabricating equipment. Striated grains further cause a high degree of variability in physical properties across a given sheet and among coils of sheets. Other alloys have a low degree of formability, an unacceptably high earing and/or undesirable strength properties.
It would be desirable to have a continuous aluminum casting process in which the aluminum alloy sheet, particularly sheet produced from 1xxx, 3xxx, and/or 5xxx series alloys, can be readily fabricated into desired objects. It would be advantageous to have a continuous casting process in which the aluminum alloy sheet has an equiaxed as opposed to a striated grain structure. It would be advantageous to have an aluminum alloy sheet that generates a low amount of finely sized particles during fabrication. It would be advantageous to have an aluminum alloy sheet that has a high degree of formability, low earing and high strength.